This invention concerns the recovery of sterilizing gas mixtures, especially mixtures in which ethylene oxide is the sterilizing gas, and in which there is included a diluent gas, such as a chlorofluorocarbon.
Ethylene oxide (ETO) has long been used to sterilize articles, especially articles such as medical implements or tools. ETO is known to kill bacteria and other harmful organisms. However, ETO is now believed to be carcinogenic. Governments have imposed regulations concerning the emission of ETO into the atmosphere, and the severity and frequency of such regulations is expected to increase.
ETO is typically diluted with a relatively inert gas such as a chlorofluorocarbon. A typical diluent gas is the substance commonly known as Freon, the latter name being a trademark of DuPont. The preferred type of Freon, in the field of the present invention, is dichlorodifluoromethane (CCl.sub.2 F.sub.2), which is sold under the trademark Freon-12. Throughout this application, the term "Freon" should be interpreted to include "Freon-12". Typical sterilizing gas mixtures include 88% Freon and 12% ETO.
Chlorofluorocarbons such as Freon have been suspected of harming the ozone layer in the atmosphere. For this reason, governments have also imposed limits on the emission of Freon into the atmosphere, and have imposed taxes or fees on Freon, to discourage its use and to encourage development of alternative refrigerants. Thus, the cost of Freon has increased considerably in recent years, and this trend is expected to continue.
In a typical sterilizing operation, only a portion of the ETO is consumed, the remainder being available for re-use, if it can be reclaimed. Because of the high cost of Freon, and also, more recently, because of the toxicity of ETO, various systems and methods have been devised for recovering spent sterilizing gas and re-using the Freon and/or the unconsumed ETO.
U.S. Pat. No. 4,954,315 shows a system and method for recovery of a sterilizing gas, and its disclosure is incorporated by reference herein. In the latter patent, the sterilizing gas is purged from the sterilizing chamber and conveyed to a condenser. The condenser is cooled by a cryogenic source, such as liquid nitrogen. The upper and lower sections of the condenser are separately cooled, by separate cryogenic cooling lines. The temperature in the upper section is sufficient to liquefy the ETO and Freon, but not the other gaseous impurities in the exhausted sterilizing gas mixture. The impurities are vented, while the liquefied ETO and Freon accumulate in the lower section of the condenser. Periodically, the liquefied mixture is vaporized and combined with fresh sterilizing gas mixture, to restore the desired proportion of ETO in the mixture. The reconstituted mixture can then be used in another sterilizing cycle. Alternatively, the ETO and Freon can be separated by distillation, and separately recovered for later use.
In the process described in the above-cited patent, the consumption of Freon is virtually zero, except for occasional losses due to leakage. Thus, it is seldom necessary to add Freon directly to the system. The ETO, however, is consumed during the sterilizing process, and therefore must be restored to the mixture, by adding ETO. The capital cost of the equipment used to practice the method is, in general, recoverable through the savings in the cost of the components of the sterilizing gas, especially the Freon. The major component of economic return is achieved either by re-use of the same Freon (i.e. by eliminating the need to purchase additional Freon), or by directly distilling and selling the recovered Freon. Some additional economic return can be derived through the recycling of ETO.
In the process of the cited patent, as in other ETO sterilization processes, the spent sterilizing gas is removed from the sterilizing chamber in several stages. This process is known as "purging" the chamber. The initial purging step removes most, but not all, of the sterilizing gas. Usually the first purging step removes about 90-94% of the sterilizing gas in the chamber. The second purging step may remove about 4-5%, and subsequent purging steps remove smaller amounts. Thus, it is usually necessary to purge the chamber at least two or three times, in order to remove most of the spent sterilizing gas. In all of the purging steps, there is also some air removed from the sterilizing chamber, along with the spent sterilizing gas. The least amount of air is removed during the first purging step, and greater amounts of air are removed during the subsequent purging steps. The latter statement is true because air is introduced into the chamber to accomplish the purging, and thus more air will be introduced during the second and subsequent purging steps.
Because the second and subsequent purging steps yield comparatively small amounts of sterilizing gas, it is relatively difficult, and expensive, to recover the sterilizing gas components by liquefying them. To recover a small amount of usable sterilizing gas, it is necessary to cool the relatively large amount of air that is mixed with the sterilizing gas. The latter fact is true because one must cool all of the gases conveyed into the condenser, including air. Cooling the air increases the amount of cryogenic coolant (such as nitrogen) required, and adds an expense to the system with no corresponding economic return. In practice, it is often necessary to provide a condenser capable of a large amount of heat removal, and to supply a large quantity of cryogenic coolant. The latter requires bulky storage facilities for the coolant.
While the process of the above-cited patent is extremely efficient and beneficial, it has disadvantages when used in environments where a relatively small amount of sterilization is performed each day. In a hospital or medical facility, for example, sterilization is done on a small scale. A hospital needs to sterilize medical implements and other pieces of equipment, but is not normally in the business of sterilizing vast quantities of material, as would be true for a manufacturer of medical supplies. Moreover, a hospital or medical facility may not have the facilities to store the large and bulky items of equipment necessary to practice the above-described process.
The system described in the above-cited patent is also quite expensive. The capital cost of the equipment used to practice the patented method is recoverable by the user, due to the savings generated by preserving and reclaiming the diluent gas (Freon), and/or by recycling ETO. However, in the case of a hospital or other small user, the rate at which the capital cost is recovered is unacceptably slow, simply because of the reduced throughput of sterilizing gas. The cost of the patented system can be recovered fairly quickly for large industrial users, which typically consume about 30,000-40,000 pounds of sterilizing gas mixture per month, but is recovered much more slowly in the case of a hospital, which typically uses only about 500-1000 pounds sterilizing gas mixture per month.
The present invention therefore provides an apparatus and method which enables a hospital, medical facility, or other relatively small user of sterilizing gas, to prevent the release of harmful ETO, and to recover the diluent and/or unused ETO for re-use. The system disclosed herein is sufficiently inexpensive to enable a small user to recover the capital cost reasonably quickly. It is also compact, and can be used with a small liquid nitrogen cylinder, instead of the massive liquid nitrogen storage tanks commonly used in industrial facilities.